The invention herein described, includes alloys which are resistant to metal dusting.
High temperature alloys which are Fe, Ni, or Co based are prone to a virulent form of corrosion known as metal dusting when subjected to environments which are supersaturated with carbon. The problem is generally encountered at temperatures ranging from 300-850xc2x0 C. Many processes of interest to the petrochemical industry which involve carbon-supersaturated environments, are limited by the lack of available reactor materials and heat exchanger materials that are resistant to metal dusting. Research has led to some understanding of the underlying mechanisms. For the Fe-based systems, the mechanism involves the initial formation of a metastable Fe3C carbide on the alloy surface in the carbon-supersaturated environments. Subsequently, graphite deposits on the metastable carbide whereby it is destabilized and decomposes to iron particles and carbon, thus triggering the corrosion process. For Ni based and Co based systems, while no metastable surface carbide forms, graphite deposition on the metal provides channels through which the metal can migrate out. In addition, carbon also supersaturates the metal and causes profuse graphite precipitation in the interior, thus leading to a breaking up of the bulk metal.
The carbon-supersaturated environment that is encountered in process streams consists of either hydrocarbon molecules or carbon monoxide. Of these, the latter is a more virulent metal dusting molecule. Heyse and coworkers have proposed carburization and metal dusting resistant coating systems that are applicable to hydroalkylation processes where hydrocarbon is the main corrosive medium. The general approach to control metal dusting is the use of alloys that can form protective surface oxide films in the environment involved. But in most currently available alloy systems, the break up of the protective surface oxide film leads to local metal dusting corrosion.
Current approaches to control metal dusting involve the use of H2S as a gas phase corrosion inhibitor, expensive high temperature alloys and tin based coatings for selected applications involving hydrocarbon corrosives (See for example, Heyse, et.al. U.S. Pat. No. 5,863,418). However, even the more expensive alloys are not fully metal dusting resistant. Coating systems, especially based on tin, have limited applications in predominantly hydrocarbon environments. The use of H2S necessitates clean up of the downstream process gas. Further, in many catalytic processes, H2S can be a catalyst poison. Thus, its use is rather limited.
Certain coating materials have been taught in the prior art. For example, see U.S. Pat. No. 5,575,902 which teaches the use of Group VIB metals, specifically chromium for coating surfaces susceptible to carburization.
What is needed in the art are materials that are highly resistant to metal dusting corrosion in petrochemical processes where supersaturated carbon environments are present.